DH had a coworker like this. DH wasn't his supervisor so he had a bit more leeway, but if you're all generally a laid-back group it could still work. They would draw the guy's attention to it and gently tease him about it. They'd say things like "Good grief, Eeyore, is everything really that bad?!" One time when I was there (not at their work, we were all hanging out), he started up and someone said "You are such a frigging grouch! I bet if you dropped your wallet and I pointed it out to you, you'd get mad at me cuz you had to bend over and pick it up!" Everybody else laughed and he looked a little sheepish. It took awhile but they basically got him to quit by forcing him to realize how often he complained.

I shared an office with a fellow like this for six years, until he retired. He had was a rant every day. Everything about school life that wasn't the classroom or his after school tutorial was a pointless distraction--sports, shows, special assemblies, pep rallies, clubs. Everything was a conspiracy against education--classroom group work, technology, the observation rubric the Board of Ed adopted, Common Core, the textbook of his supervisor's choice. Any minor criticism of his teaching (he was an excellent teacher, but sometimes observers are looking for something you don't believe in doing) was an attempt to force him out, since he was a senior faculty member with a high salary. I never did come up with a solution to get him to shut up. All I did was say "Uh huh" in as bored a tone as possible until he wound down. Since I wasn't his supervisor, I couldn't very well talk about his effect on the rest of the team.

To clarify, I'm *a* supervisor, but I'm not *his* supervisor. His own supervisor knows about the issue but doesn't particularly care. He hides in his office all day and doesn't really have to be around the complainer much.I, on the other hand, spend zero time in my office and all my time out on the shop floor so I hear it constantly.

That's kind of why I wasn't sure which section to put this in. It happens at work and affects my and my co-workers' performances by being a real bummer, but in the work sense, I have no authority over him really. So, it is work-related and it's not...I feel like I can't order him to stop complaining, not officially, but one-on-one, person-to-person, I would like to talk to him about his attitude and how it affects everyone around him.

I've had varying success with some sort of take on "Geez, Negative Nancy, way to bring us all down!" In one case, it made everyone laugh and the offender quickly changed - in another, the offender changed for a little while but quickly went back to his old ways. But if you have a relaxed workplace generally, or a friendly atmosphere where you can joke around a bit, I'd try something like that as you're not his supervisor.